1. Field of the Invention
This invention relates to wet oxidation of aqueous wastes, and, more particularly, to methods for controlling the operating temperature of wet oxidation processes.
2. Description of the Prior Art
Wet oxidation, which is used to treat aqueous waste containing combustible organic and inorganic materials, involves the reaction of an oxygen-containing gas with these materials at elevated temperatures and pressures. Various components in raw waste oxidize at different rates, so in one mode of operation an oxygen-containing gas is mixed with the raw waste and the resultant influent introduced into a reaction vessel where the mixture is retained under elevated temperature and pressures for a sufficient time for the oxidation reactions to take place. The oxidation reaction generally is exothermic and some form of heat exchange typically is used to remove heat from the oxidized waste or reaction vessel effluent and preheat the influent before being introduced into the reaction vessel. To maximize the effectiveness of the heat transferred from the effluent to the influent, the oxygen-containing gas can be mixed with the raw waste prior to the preheating step.
In some cases, a highly reactive waste can begin to oxidize prior to the influent entering the reaction vessel, usually during or shortly after preheating. The resulting temperature fluctuations of the influent makes it difficult to control the steady state or operating temperature in the reaction vessel so that the desired oxidation reactions can take place without excessive temperature and/or pressure build up. Also, during the start-up of some wet oxidation processes, it is advantageous to reach steady state conditions within a relatively short time.
Steam heat exchangers have been used for initially heating the influent to a wet oxidation reactor during start-up. This is disclosed in Zimmerman U.S. Pat. No. 2,903,425 issued Sept. 8, 1959 and Hoffman U.S. Pat. No. 3,876,497 issued Apr. 8, 1975. Also, Grotz U.S. Pat. No. 4,624,842 issued Nov. 25, 1986 and Grotz U.S. Pat. No. 4,744,966 issued May 17, 1988 disclose using superheated steam to cool that effluent from an exothermic reaction.
Applicant is unaware of any prior art wet oxidation processes where a heat exchanger and superheated steam are used to initially heat the influent stream during the start-up of the process and then used to cool a preheated influent to a temperature desired for steady state operation.